In today's radio communications networks a number of different technologies are used, such as Long Term Evolution (LTE), LTE-Advanced, 3rd Generation Partnership Project (3GPP) Wideband Code Division Multiple Access (WCDMA) system, Global System for Mobile communications/Enhanced Data rate for GSM Evolution (GSM/EDGE), Worldwide Interoperability for Microwave Access (WiMax), or Ultra Mobile Broadband (UMB), just to mention a few.
LTE is a Frequency Division Multiplexing technology wherein Orthogonal Frequency Division Multiplexing (OFDM) is used in a downlink (DL) transmission from a radio base station to a user equipment (UE). Single Carrier—Frequency Domain Multiple Access (SC-FDMA) is used in an uplink (UL) from the user equipment to the radio base station. Services in LTE are supported in the packet switched domain.
In a time domain, one subframe of 1 ms duration is divided into 12 or 14 OFDM, or SC-FDMA, symbols, depending on a configuration of the subframe. One OFDM or SC-FDMA symbol comprises a number of sub carriers in the frequency domain, depending on a channel bandwidth and configuration. One OFDM or SC-FDMA symbol on one sub carrier is referred to as a Resource Element (RE).
In LTE no dedicated data channels are used, instead shared channel resources are used in both downlink and uplink. These shared resources, Downlink Shared Channel (DL-SCH) and Uplink Shared Channel (UL-SCH), are each controlled by one scheduler in the radio base station, which scheduler assigns different parts of the downlink and uplink shared channels to different user equipments for reception and transmission respectively.
The schedulers are in full control of in which subframe a user equipment should receive a DL-SCH transmission and which subframe the user equipment is allowed to transmit on UL-SCH. Scheduling decisions are sent to the user equipment as downlink assignments and uplink grants. Downlink assignment information and uplink grants are transmitted in Downlink Control Information (DCIs) using L1/L2 control signaling. A downlink assignment message indicates if there is data to be received for the user equipment on the DL-SCH.
For an UL transmission, a bandwidth resource assigned to one user equipment is always a set of contiguous scheduling blocks (SBs) due to the constraints of the SC-FDMA transmission scheme. The bandwidth resources are indicated in the DCI by a start SB and an allocation size in number of SBs. LTE supports full dynamic scheduling, which means that the bandwidth resource assigned to the user equipment is set to be valid only for one subframe. In a next subframe, the same bandwidth resource may be allocated to a different user equipment.
Dynamic scheduling enables multiple user equipments to share all, or parts of, available frequency resources in one subframe; all, or parts of, frequency resources are assigned to one user equipment; and no user equipments are allocated any frequency resources.
A resulting resource allocation over time and frequency depends both on properties of the user equipments in the system, i.e. the number of user equipments, traffic models of the user equipments, radio channel characteristics, and an algorithm implementing a scheduling functionality. A strategy defining in which way resources in time and frequency are allocated to a set of user equipments is commonly referred to as a scheduling algorithm.
The choice of scheduling algorithm gives rise to different behavior of a radio communications network and how a user of a user equipment experiences performance of the radio communications network. One scheduling algorithm may prioritize fulfilling delay constraints of data traffic, another scheduling algorithm may prioritize to let user equipments located near the cell center experience peak bit rates, whilst a third scheduling algorithm may share resources in time and frequency as fair as possible among the user equipments in the cell. That is, given a specific scenario of a radio communications network, i.e. characteristics of the user equipments, cell sizes and fading environments etc., different scheduling algorithms may give different performance of the radio communications network.
A fairness of a scheduling algorithm is a measure based on a performance of a user equipment located near a cell edge of a cell, also referred to as a cell edge user equipment, compared to a capacity of the cell serving the user equipment. It is often defined as the 5th or 10th percentile user bit rate compared to an average user bit rate in the cell. Scheduling algorithms that prioritize user equipments that have a good channel condition perform so called a channel dependent scheduling. The channel dependent scheduling utilizes a multi user equipment diversity, where multiple user equipments are spread out in the cell and thus the user equipments experience different channel quality dips at different frequencies and at different times. A pure channel dependent scheduling algorithm always prioritizes the user equipment that has a good radio condition. A result is that a throughput of the cell will be maximized; however user equipments in bad channel conditions may be starved. The pure channel dependent scheduling is therefore said to be unfair.
Proportional fair (PF) scheduling adds control of an overall fairness in the radio communications network by prioritizing user equipments not only on based on a channel quality of the user equipment but also on a rate of a transmission. The overall fairness of the scheduling is controlled by steering a proportion of the two components, i.e. instantaneous channel quality and an average rate of flow. PF scheduling is implemented by prioritizing the user equipments using a weight function W defined asW=(1−CQF)·WR+CQF·WCQ whereWR is a weight depending on the rate of a flow,WCQ is a weight depending on the channel quality, andCQF stands for a Channel Quality Fraction (CQF) parameter that controls a relation between the two weight components, i.e. proportion between channel dependency and rate in the scheduling weight.
The PF scheduling strategy is able to utilize channel variations to improve overall cell throughput while still ensuring the fairness between UEs.
As previously described, LTE enables dynamic scheduling where resources are orthogonal in frequency domain enabling channel dependent scheduling to be used in both time and frequency. To prioritize which resources in frequency domain that should be allocated to a UE is called Frequency Selective Scheduling (FSS). If applied in an LTE scenario, an optimal frequency selective scheduler would only assign resources to a UE where a gain-to-interference ratio (GIR) is high. UEs may report to the radio base station CQI reports based on measurements of SINR on downlink reference signals of known power. The radio base station then calculates GIR corresponding to the received measurements of SINR. A higher GIR indicates a better subband and is defined as a signal power gain over interference.
One way of implementing FSS is to implement proportional fair scheduling, where the channel quality measure is based both on time and frequency variations, that is, proportional fair in time and frequency. PF in time and frequency is believed to ensure higher cell throughput and fairness among UEs.
Frequency selective scheduling algorithms solutions of today require immense computation power to find an optimized solution. One example of such solutions is to compute number of bits as the channel quality measure. That requires performing link adaptation before making scheduling decision which is connected to very high computational complexity. A simple channel quality metric is crucial to realize frequency selective scheduling in reality. Typical channel quality measures, which are more feasible for implementation, are Gain to Interference and Noise Ratio (GINR) or Signal to Interference and Noise Ratio (SINR). However, methods using both these typical channel quality measures have associated problems. Using GINR, the scheduler in the radio base station tends to favour UEs in a centre of the cell. SINR is a good channel quality measure, but it is hard to estimate SINR before knowing where and how many resource blocks are allocated.